1. Field of the Invention
The present invention relates to a method for controlling an AC motor which facilitates detection of the primary resistance of the AC motor with excellent accuracy, and more particularly to a method for controlling an AC motor which facilitates detection of a malfunction by comparing the estimated primary resistance value of each phase.
2. Description of the Prior Art
Magnetic flux and torque of an AC motor can be controlled by resolving a current flowing through the AC motor into a magnetizing current component parallel to the magnetic flux and a torque current component perpendicular to the magnetic flux, and by controlling the magnetizing current and the torque current components independently. It is well known that speed of the AC motor can be controlled highly accurately by the so-called transvector control method that treats the current of the AC motor as a vector consisted of two components.
According to this method, the AC motor is controlled by putting an equivalent-circuit of the AC motor as a control model. Control constants based on the equivalent circuit constants are then pre-set to a control apparatus. Usually, the design values of the motor constants are used as the control constants. However, there are differences between the design and actual values of the motor constants, which cause control errors. Variations of the motor constants during operation of the motor also cause control errors. Furthermore, the transvector control method cannot be applied when the constants of the motor already in service are unknown.
In the transvector control method, induced electromotive force which relates to the magnetic flux must be determined. The induced electromotive force is calculated by subtracting the voltage drop attributable to the internal impedance of the motor from the terminal voltage of the motor. The voltage drop attributable to the internal impedance of the motor is obtained by multiplying the magnetizing current component and a primary resistance value of the motor. A drawback of the transvector control method is that generated torque becomes insufficient, especially when the motor is driven at low speeds, because an appropriate output reference voltage value cannot be calculated when the primary resistance value is different from the pre-set value.
To solve this problem, a method to obtain the primary resistance value by dividing output voltage by a phase current of predetermined value flowing through the motor has been proposed. (See RM-85-26, Technical Meeting Paper on Rotating Machine, p. 65, Institute of Electrical Engineers of Japan.) This method, however, requires high precision detectors. Further, since the primary resistance value of a large capacity motor is small, the high precision detectors would not improve accuracy of the measurements. Moreover, in the transient period, induced electromotive force by magnetizing inductance is added to the voltage drop attributable to the primary resistance. Since decay time of the induced electromotive force is prolonged in an AC motor having a large secondary time constant, e.g., a large capacity AC motor, the resistance value obtained by dividing output voltage by a current requires time to reach a true primary resistance value. Additionally, the motor windings and/or the semiconductor switching elements of the power converter are different depending on a phase angle of voltage applied to the AC motor. Therefore, the resistance values of the motor windings and/or the semiconductor switching elements vary. Because of these variations, the measured primary resistance value varies depending on the phase angle difference, further causing control errors.
In view of the foregoing, an object of the present invention is to provide a control method that measures the primary resistance of an AC motor with high accuracy in a short time.
It is another object of the present invention to provide a control method that measures, without using high precision detectors, the primary resistance of an AC motor with high accuracy in a short time by eliminating the errors caused by the phase angle difference.
It is yet another object of the present invention to provide a control method which facilitates detection of malfunctions such as breaking of wiring, open phases and three-phase unbalance of the AC motor or the power converting means.